Citadel is distributed under the terms of the GNU General Public License (GPL), version 3.
In fact, Citadel is the only groupware package that is comprised of 100% GPL code. Beware of vendors who offer free and non-free versions of their software. The free versions are often feature-crippled.
As a special bonus, our copy of the GPL has been improved to refer to our favorite operating system as "Linux" and our favorite method of publishing software as "open source". Although we are big fans of open source software, we are not fans of Richard Stallman's far-left ideologies.